An optical distributed acoustic sensing (DAS) system uses an optical waveguide, such as an optical fiber, as a distributed sensor to measure the properties of acoustic waves that strain the waveguide. This sensing is performed by interrogating the backscattered light returning from the waveguide. Changes in the backscattered light can indicate not only the presence of acoustic waves, but also certain characteristics of the acoustic waves.
During hydrocarbon recovery operations, including well drilling, it is common to use DAS systems to acquire vertical seismic profile (VSP) data along the length of a wellbore. Such VSP data can include thousands of channels of seismic data, and it is sometimes desirable to downsample the number of channels of data by a factor of two or more. However, due to the nature of the fundamental physics of the measurement method, the data may be subject to noise that corrupts the spatial consistency of the seismic measurements across adjacent channels. This noise can be caused by various sources, such as external sources (e.g., environmental disruptions, surface acoustic waves, nearby drilling), time-varying noise sources within a fiber (e.g., polarization fading or Rayleigh fading), and increased noise levels near the waveguide termination. The resulting variations in the measured characteristics of the acoustic waves which impinge on the waveguide make it difficult to interpret and properly downsample measurements made by the DAS system.
Thus, it will be appreciated that improvements are continually needed in the art of interpreting DAS data collected in conjunction with subterranean wells. Such improvements could be useful for evaluating data collected from well acoustic sensing systems other than optical DAS systems, such as well acoustic sensing systems which include arrays of multiplexed point sensors (e.g., fiber Bragg gratings), or non-optical distributed acoustic sensing systems.
While embodiments of this disclosure have been depicted and described and are defined by reference to exemplary embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure.